This invention concerns membrane filtration in sewage treatment facilities, particularly with flat-plate membranes, and is directed to removal of fouling in the form of caked, dewatered sludge from the surface of the membranes in an efficient manner.
Membrane equipment used in submerged membrane bioreactors (sMBRs) operates in what is called mixed liquor, a concentrated solution of active biosolids. In this environment membranes are subjected to several types of fouling, most of which occur at the microscopic level. However, a type of fouling called dewatering, caking or sludging can occur under various conditions at the macroscopic level and significantly decrease membrane performance. This invention is concerned with the latter type of fouling, which is on a macroscopic level, whereby solids accumulate at the membrane surface to form a dense and visible cake. This type of phenomenon generally occurs in pockets as a result of insufficient rise velocities in the presence of high solids and debris. In hollow-fiber systems that use potting to secure individual fibers, solids accumulation will often occur at the potted ends. For flat-plate systems, solids can accumulate between membrane plates or cartridges supported by frames called cassettes.
Theoretically, the likelihood of dewatering is a function of flux and rise velocity, among other factors. This is one of the reasons some membrane suppliers have moved to reduce design fluxes. However, in practice, the root cause of dewatering is often related to system design and operational issues that are unrelated to membrane equipment. It is a fact for any sMBR system that an operator can accidentally (manually) turn off air during filtration or that plant hydraulics can starve specific membrane zones creating de facto thickeners. The problem that needs to be addressed is how to handle dewatering if and when it occurs.
Until recently there were no known methods for addressing dewatering without manual or physical cleaning (by hand) at the membrane module/element (cartridge or fiber bundle) level. Water jetting with fire hoses or garden hoses has been used unsuccessfully and is not recommended by most equipment manufacturers because it can damage membrane materials or joints (connection points). Also, water jetting has only been used at the cartridge level, cleaning one cartridge at a time, making costs and treatment time prohibitively high.
It is an object of this invention to efficiently clean caked sludge from flat-plate membrane cartridges using a liquid spray that does not damage the cartridges.